1. Field of the Invention
This invention relates to a control device for differential limiting mechanisms of a four-wheel drive car and, more particularly, to a device for controlling the differential limiting mechanisms in a four-wheel drive car provided with a central differential gear having a first differential limiting mechanism, a differential gear having a second differential limiting mechanism incorporated in one of a drive shaft connected with front wheels and a drive shaft connected with rear wheels and a differential gear incorporated in the other of said drive shafts.
2. Description of the Prior Art
When a vehicle travels in such case that one of left and right driving wheels of the vehicle is on a road surface having low coefficient of friction and the other is on that having ordinary coefficient of friction, the wheel on the road surface having the low coefficient of friction may slip, i.e., may be put under a wheel spin condition. Since driving force is not generated in the other driving wheel when this wheel spin occurs, it is necessary that the differential is limited to ensure traction.
In a four-wheel drive car, the following art has been proposed in which when a difference between revolutions or rotational frequencies of front and rear wheels exceeds a predetermined value, the differential of a central differential gear is limited for a certain period of time and when a difference between rotational frequencies of left and right rear wheels exceeds a predetermined value, the differential of a differential gear of a rear shaft is limited for a certain period of time, and released thereafter from the limitation (Autocar; pages 21-23, Feb. 26, 1986).
A differential control device which has been separately proposed (Patent Application Ser. No. 62-64300) is a device which controls the differential of a differential gear in a vehicle for driving left and right front wheels or left and right rear wheels with the differential gear having a differential limiting mechanism, and comprises means for detecting respective rotational frequencies of the left and right front wheels and left and right rear wheels, a controller for receiving signals from the respective detecting means and means for operating said differential limiting mechanism, wherein said controller controls said operating means such that the differential of said differential gear is limited according to a difference between the rotational frequencies of the left and right wheels driven by said differential gear and said differential gear is released from limitation of the differential according to a difference between the rotational frequencies of the front and rear wheels.
According to the prior art in relation to the four-wheel drive car noted above, when the differential is limited for a certain period of time and then released from limitation, the operation for limiting the differential again when the difference between the rotational frequencies exceeds a predetermined value and then releasing from the limitation is repeated. Therefore, sounds and vibration are generated from operating instruments every time the operation noted above is repeated. Besides, the behavior of vehicle varies intermittently. Thus, travelling feeling may be damaged and the durabilities of the operating instruments are degraded.
According to the art which has been proposed separately is effective in such case that one of the left and right drive wheels is put under the wheel spin condition or the like. However, this art does not cope with the difference between the rotational frequencies of the front and rear wheels.